Patterning of an amine-terminated nanolayer by extreme ultraviolet
Sangwoon Moon, Sukmin Chung, Cheolho Jeon, Chong-Yun Park, Han-Na Hwang, Chan-Cuk Hwang, Hajin Song, and Hyun-Joon Shin
Citation: Applied Physics Letters 91, 193104 (2007); doi: 10.1063/1.2803216 View online: http://dx.doi.org/10.1063/1.2803216
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/91/19?ver=pdfcov Published by the AIP Publishing
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Patterning of an amine-terminated nanolayer by extreme ultraviolet
Sangwoon Moon and Sukmin Chung
Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea Cheolho Jeon and Chong-Yun Park
Department of Physics, Sungkyunkwan University, Suwon 440-746, Korea
Han-Na Hwang, Chan-Cuk Hwang,a兲Hajin Song, and Hyun-Joon Shin Beamline Research Division, Pohang Accelerator Laboratory, Pohang 790-784, Korea
共Received 24 April 2007; accepted 6 October 2007; published online 6 November 2007兲
The adsorption of NH3 molecules on the Si共100兲2⫻1 surface constructs a cleaner and more well-defined amine layer than self-assembled monolayer such as aminosilylated layer, which make it possible to study photoinduced reactions between amines and monochromatic light with shorter wavelength than ultraviolet, i.e., extreme ultraviolet and soft x ray. We report that the molecular layer of amine groups reacts with extreme ultraviolet and soft x ray, which can be used to make fine patterns on the amine-terminated layer. The amine patterning with the leading postoptical lithography using extreme ultraviolet could be applied to fabricating future molecular nanodevices. © 2007 American Institute of Physics.关DOI:10.1063/1.2803216兴
Amine group-terminated surfaces have been used as sub- strate for fixating nanoparticles1 and biological molecules, such as DNA,2 enzymes,3 and antibodies,4 and finally, they can be applied to fabricating molecular nanodevices. Pattern- ing amine layer in micro- or nanoscale is a key process to reach the final goal. Ultraviolet lithography共UVL兲has been a standard tool in patterning process for several decades.
However, its ability to make nanopatterns on a wafer is now limited to a feature size of 45 nm.5In order to overcome the limit, extreme ultraviolet lithography共EUVL兲has been pro- posed as a leading postoptical lithography.6,7Therefore, the interaction of amines with EUV is a fundamental issue for making future molecular nanodevices based on EUVL.
Polymer films with a thickness of micrometer to several hundreds of nanometers have been employed as photoresist 共PR兲for manufacturing practical devices in UVL.8However, their application to EUVL has significant drawbacks such as shallow penetration depth of EUV by high optical absor- bance, blur of patterned images due to smaller depth of focus than PR thickness and due to back scattering of electrons at PR surface.9Thin polymer films with a thickness of tens of nanometers or self-assembled monolayers 共SAMs兲 with monomolecular thickness have been suggested as a new plat- form to overcome these problems. Some research groups re- ported their possibility as PR and practical applications in lithography.8,10 These monomolecular layers are formed by immersing a substrate into a solution of surface-active mate- rials or by spin coating. To our knowledge, impurities in solution and atmosphere can cause several undesired prob- lems such as nonuniform density, irregular orientation, and height fluctuation during the formation. From long experi- ence in surface science field, the adsorption of NH3 on the Si共100兲2⫻1 surface is well known to construct uniform and structurally well-defined amine groups, because the mol- ecules adsorb softly on the underlying crystalline surface, where broken bonds are regularly distributed. Therefore, this system was chosen as a model system in this work to study the interaction between amines and EUV. The clean and well-ordered amine layer was observed to react with mono-
chromatic light with shorter wavelength than UV, i.e., EUV and soft x ray. We used EUV near the 13.4 nm wavelength, which is being applied to practical EUVL in device industry.
The EUV-induced surface modification at this wavelength has not been reported for the NH3/ Si, amine-terminated SAMs, and other representative well-defined molecular sys- tems, such as H / Si共Ref.11兲and Cl/ Si.12We report that the surface modification can be used to make fine patterns of amine groups.
Figure 1 is a schematic diagram of the amine groups patterning on the Si共100兲surface. First, the Si共100兲 sample was put inside the chamber with a base pressure of ⬃2
⫻10−10Torr. The sample was directly heated up to 1250 ° C by passing dc through it and the temperature was maintained for a few seconds to remove the native SiO2layer and some
a兲Electronic mail: [email protected]
FIG. 1.共Color online兲 关共a兲–共f兲兴Schematic illustration of the patterning of an amine-terminated nanolayer through an EUV-induced surface modification.
Structural models and Si 2pcore level spectra of共g兲the bare Si共100兲2⫻1 and共h兲the amine nanolayer on the Si共100兲surface.
APPLIED PHYSICS LETTERS91, 193104
共
2007兲
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carbon contaminants. This procedure was repeated under 2
⫻10−9Torr to prevent carbon and oxygen contaminations by residual gases inside the chamber. After the heating proce- dures, the clean Si surface was obtained. Then it was ex- posed to ammonia共NH3兲gas of 10 L共for 100 s at the partial NH3 pressure of 10−7 Torr, 1 L = 10−6Torr s兲 to make the ammine layer at room temperature关Fig.1共a兲兴. At room tem- perature, NH3molecules are dissociated into amine 共–NH2兲 and hydrogen共–H兲, then the dissociated species bond easily with the reactive dangling bonds revealed on the clean Si共100兲surface.13As a consequence, a thin amine-terminated layer is formed on the Si共100兲surface关Fig.1共b兲兴. The clean and ammine terminated surfaces were confirmed by synchro- tron radiation photoemission spectroscopy 共PES兲. After the base pressure was recovered, EUV or soft x ray was irradi- ated on the amine共–NH2兲 nanolayer. Here, two irradiation methods were used; projection printing by irradiation through a gold mesh 共hole of 250⫻250m and wire of 20m thickness兲, as shown in Fig.1共c兲, and direct writing to make arbitrary patterns using focused beam关Fig.1共d兲兴. In direct writing, EUV共= 1.9 nm兲 is focused into⬃1m to pattern one pixel at a given time. The amine-terminated layer is chemically transformed into silicon nitride on EUV- exposed regions.14Different chemical properties between ex- posed and unexposed regions make a chemical pattern on the surface 关Figs. 1共e兲 and 1共f兲兴. Surface core level shift was employed to confirm whether the clean and amine- terminated surfaces were realized in these experiments. The most striking characteristic of the Si共100兲 surface is asym- metric dimers with reactive broken bonds关Fig.1共g兲兴.15Since up dimers 共purple atoms兲have rich valence electrons, their Si 2p core level 共S兲 appears at a lower binding energy of
⬃500 meV than the bulk peak共B兲.16The asymmetric dimers become symmetric as the broken bonds are saturated with the amines and the hydrogens, as shown in Fig.1共h兲,13where the structures are slightly exaggerated for simplicity. Actu- ally, the building block in the side view and their inverse structure for the plane indicated with dashed line are ran- domly distributed at room temperature. The Si 2ppeak from the Si dimers bonded with nitrogen 共denoted by N1兲 is clearly separated from the bulk peak due to the large differ- ence in electronegativity between silicon and nitrogen. The strong N1 component indicates that Si– NH2 was widely formed on the surface.
All experiments in this paper were carried out in UHV environment. The base pressure of the system was ⬃2.0
⫻10−10Torr and background gases such as H2O, H2, CO2, and CO were maintained below 5⫻10−11Torr, as confirmed by a residual gas analyzer. The chamber pressure was in- creased to ⬃9⫻10−10Torr during the irradiation through a Au mesh and⬃5⫻10−9 Torr during the focused beam irra- diation. Gases desorbed from the Au mesh, pin hole, or zone plate in front of the sample may be responsible for the in- crease of the chamber pressure.
The amine-terminated layer was exposed to EUV 共
= 1.9⬃11.8 nm, photon flux of 3⫻1012photons/ s cm2兲 from the 7B1 bending magnet beamline in Pohang Accelera- tor Laboratory for 1 h, but no change was observed. This is a natural consequence if we consider that EUV or soft x ray is generally used to analyze solid surface without destroying its state. The amine-terminated nanolayer then was exposed to highly brilliant EUV of = 11.8 nm 共photon flux of 2
⫻1014photons/ s cm2兲 from the 8A1 undulator beamline
through a gold mesh for 1 h关Fig.1共c兲兴. Scanning photoelec- tron microscopy共SPEM兲 images for Si 2p, N 1s, and O 1s core levels show that mesh patterns were formed on the sur- face关Fig. 2共a兲兴.-PES were recorded at both exposed and unexposed regions. Notice that the N 1s core level binding energy is lowered by ⬃0.7 eV, indicating that the amino group 共–NH2兲 is transformed to silicon nitride upon EUV irradiation.14A similar experiment was also performed at a shorter wavelength of = 1.9 nm. A Fresnel zone plate was used to increase the photon flux per unit area by focusing monochromatic beam into⬃1 m2. Figure 2共b兲 shows the patterns fabricated by the focused EUV. Desired patterns could be directly written on the substrate by the fine control of the sample stage, as shown in Fig.1共d兲. In this case, two characters, U7 standing for the undulator with 7 cm period- icity, were written. SPEM images for Si 2p, N 1s, and O 1s core levels clearly show the letters written on the molecular nanolayer. -PES was used to analyze the chemical differ- ences between the irradiated and unirradiated regions. The N 1score level shift of⬃0.7 eV was also observed. Thus, the bright regions in the N 1s image reflect that silicon nitrides were formed at the exposed regions. The appearance of the adsorbate-induced component in the Si 2pspectrum and the increase of the O 1speak after irradiation indicate that sili- con nitrides and oxides共or oxynitrides兲were formed on the regions exposed to EUV probably due to residual H2O inside the chamber. As shown in the structural model of Fig.2共c兲,
FIG. 2. 共Color online兲SPEM images of fine patterns and-PES spectra obtained from exposed共blue dot in SPEM image兲and unexposed regions 共black dot in SPEM image兲for共a兲= 11.8 nm and共b兲= 1.9 nm. The PES spectra from exposed and unexposed regions are represented by blue solid line and black dot, respectively.共c兲Atomic model of the boundary between exposed共right side兲and unexposed共left side兲regions as marked with red rectangle.
193104-2 Moonet al. Appl. Phys. Lett.91, 193104共2007兲
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the N–H bonds of the amines, which act as a passivation layer against the chemisorption of adsorbates, may be broken by the EUV in an intermediate step, after which the unstable nitrogen atoms form silicon nitrides and water molecules are adsorbed on the revealed broken bonds.
In order to understand how EUV influences on the other dissociated species, Si–H, synchrotron radiation was irradi- ated on the silicon monohydride.11 We could not find any change induced by monochromatic photons with wavelength of 1.9 to 11.8 nm generated from the undulator. Therefore, the Si–H species on the NH3-adsorbed Si surface seem not to be affected by EUV irradiation. They act as only spacers defining the distance between the amines. The distance may be also controlled by changing substrate orientation or by the surface treatment suggested by Finstadet al.17
The surface modification by EUV or soft x ray is not a general phenomenon that applied to every molecular system.
While the destruction of SAMs or polymer layers frequently has been an issue,18simple molecules on silicon surface are known to be stable for the EUV or soft x-ray irradiation.
Other well-ordered molecular nanolayers, such as H / Si and Cl/ Si, show different behaviors upon EUV or soft x-ray ir- radiation. The hydrogen atoms on the H / Si surface are well known to be desorbed with scanning tunneling microscope 共STM兲tip,19electron gun20or UV light,21while EUV or soft x ray makes no effect on the surface.11On the other hand, the Cl/ Si surface is affected by STM tip,22 electron gun,23 UV light,24or focused soft x ray,12but no change was observed upon unfocused EUV irradiation near 13.4 nm. The EUV- induced amine group modification near 13.4 nm has not been reported for amine-terminated SAMs, other well-defined mo- lecular nanolayers, such as H / Si共Ref.11兲and Cl/ Si,12 and even the same NH3/ Si system. The well-defined structure of the present system and brilliant light source make it possible to observe clearly such results.
SPEM was used in this work to image chemical patterns induced by EUV irradiation. Because of its restricted spatial resolution, the feature size was in micrometer scale25 and thus the resolution must be improved for practical nanofab- rication. A variety of methods are currently being developed to fabricate nanopatterns using EUV共or soft x ray兲without complex and expensive lithographic tools, i.e., steppers and scanners. Considerable effort has been made to focus EUV or soft x ray into nanometer scale. Recently,⬃15 nm focus- ing was achieved by using the zone plate lens.26 Owing to the development of the EUV focusing technique, direct writ- ing in nanometer scale or medium volume production of fine patterns is possible through the zone plate array lithography.27On the other hand, EUV interference lithogra- phy is applicable to fabricating patterns of regular shape such as nanowire and nanodot by using Lloyd’s mirror or trans- mission grating.28,29 The functional group patterning com- bined with these frontier techniques will provide consider- able usefulness in surface functionalization and therefore could be applied in the manufacture of future molecular nan- odevices. Since our system is intrinsically ultrathin, consist- ing of one or two atomic layers, better resolution can be expected when it is patterned with these techniques.
In summary, we report that the well-defined amine layer on the Si surface is chemically transformed by EUV or soft x ray with a shorter wavelength than UV. The EUV-induced surface modification was used to make fine patterns on the amine-terminated nanolayer. The patterned surface could be applied as a template for future molecular devices.
This work was supported by grants funded by Ministry of Commerce, Industry and Energy of Korean government 共Self-Assembled Nanostructure on the Nanopatterned Sur- face关10022918-2006-22兴 and the Program for the Training of Graduate Students in Regional Innovation关TH-19兴兲and a Korea Research Foundation Grant共KRF-2006-312-C00513兲.
Experiments at PLS were supported in part by the Korean Ministry of Science and Technology 共MOST兲 and Pohang University of Science and Technology共POSTECH兲.
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